Critically evaluate and characterize the incidence and types of tumors which occur in a population of aging rhesus (Macaca Mulatta) monkeys. Characterization of tumors shall include histopathologic and electron microscopic evaluation, cytogenetic studies (peripheral blood, bone marrow, and tumor cells), virologic evaluation of tumor specimens, and transmission studies using newborn monkeys as recipients. Animals in the group shall be periodically monitored by complete physical examinations and hematologic evaluations. Selected high-risk animals shall continue to be used in a breeding program initiated to evaluate the possible genetic damage in progeny of such animals as well as determine the incidence of leukemia or solid tumors in infants with such a parental background.